How do I reduce / remove them? This type of artifact is common in old concert videos as far as I know.

I tried WDeflicker and it would be a good way to at least reduce them a bit but it introduces dark lines just as Osmiridium warned - "If your values are too strong dark horizontal lines may appear in scenes with high contrast." Perhaps there is another way which I just can't think of? Thanks very much in advance to whoever replies!